MemoryUsage just returns a constant if the argument is a word

[Stefan-Kohl]

Observed behaviour

gap> F := FreeGroup(3);;
gap> w := One(F);;
gap> for i in [1..10000] do w := w * Random(GeneratorsOfGroup(F)); od;
gap> MemoryUsage(w);
40
gap> MemoryUsage(ExtRepOfObj(w));
120248

Expected behaviour

gap> F := FreeGroup(3);;
gap> w := One(F);;
gap> for i in [1..10000] do w := w * Random(GeneratorsOfGroup(F)); od;
gap> MemoryUsage(w); # approximately!
120248

Copy and paste GAP banner (to tell us about your setup)

┌───────┐ GAP 4.8.4, 04-Jun-2016, build of 2016-06-10 15:51:35 (CEST)
│ GAP │ http://www.gap-system.org
└───────┘ Architecture: x86_64-pc-linux-gnu-gcc-default64
Libs used: gmp, readline
Loading the library and packages ...
Components: trans 1.0, prim 2.1, small* 1.0, id* 1.0
Packages: GAPDoc 1.5.1

[ChrisJefferson]

The problem is that the object isn't a IsPositionalObjectRep, but is really (the data is stored in ![1]). I'm not sure what exactly is going wrong, and where this should be fixed.

[frankluebeck]

According to section ?NewRepresentation it seems to be a bug that the representation IsBLetterAssocWordRep is not a subrepresentation of IsPositionalObjectRep. After changing the declaration (in wordass.gd) to

DeclareRepresentation( "IsBLetterAssocWordRep", 
                        IsLetterAssocWordRep and IsPositionalObjectRep, [] );

MemoryUsage works as expected in the above example.

There are probably other similar bugs in that file. It would be good if someone could have a look who knows more about these various representations than me.

[stevelinton]

We should be able to scan for representations that are not descended from IsInternalRep, IsPositionalObjectRep, IsComponentObjectRep or IsDataObjectRep.

[markuspf]

We have:

• IsSyllableAssocWordRep
• IsLetterAssocWordRep
• IsBLetterAssocWordRep
• IsWLetterAssocWordRep
• IsPermGroupGeneralMapping
• IsSubgroupOfWholeGroupByQuotientRep
• IsGenericCharacterTableRep
• IsNullMapMatrix
• IsDefaultGeneralMappingRep
• IsInverseGeneralMappingRep
• IsMemberPcSeriesPermGroup
• IsSlicedPerm
• IsSlicedPermInv
• IsFromTheLeftCollectorRep
• IsGroupClassByListRep
• IsNilpotentLieAutomorphismRep

(Found by filtering AllRepresentations() by not having any of the four Reps above in their IMPL_FLAGS, AllRepresentations() being a function I just added to the library).

[fingolfin]

Note: I removed my last two comments in favor of a separate issue (see #1043). Should have done that from the start. Sorry for the noise.